Wallboard is generally installed in large panels, which are nailed, screwed, or glued to the studding of walls of buildings. The joints where sections of the wallboard are butted together are covered with a joint compound and then a fiberglass or paper reinforcing tape is embedded within the joint compound and then permitted to dry. When the joint compound is dry, a second application of the joint compound is applied over the joint and is permitted to dry. A coating of the joint compound is also applied to cover nail heads or screws or any cracks in the wallboard and let dry. After the joint compound dries, the joint and covering of the nails or screws are lightly sanded and the wall is then finished with decorating material such as paint.
Typically, tape joint compositions, which are normally referred to as joint compounds, contain a binder, a thickener system, a filler, water, a biocide, clay and mica. This joint composition is a ready-mix, drying type composition that is normally sold in five-gallon cans or corrugated boxes. The water and filler are the ingredients that comprise the largest weight percentage in the joint composition. Joint compounds are either regular weight compounds that are the traditional type or lightweight compounds. The regular weight joint compounds have a weight of about 13 to 14 pounds per gallon (ppg) (1.55–1.65 g/cc) while the lightweight joint compounds have a weight of about 8 to 10 ppg (0.9–1.2 g/cc.) The lightweight joint compound is becoming the more desirable compound used in the industry.
Although attapulgite clay is the clay of choice for tape joint compound rheology control, it imparts to the tape joint composition many undesirable properties. Attapulgite clay (1) is a key factor causing rheology and viscosity instability with aging of the joint compound, (2) can contribute to cracking upon drying, (3) can give a mealy, grainy texture to the tape joint composition requiring repeated troweling to obtain a smooth surface on the wall, (4) is a natural product requiring either lot-to-lot standardization by the user or constant changes in water demand and clay use level due to its natural variability, (5) causes variability in extent of shrinkage upon drying, especially when use levels have to be altered due to lot-to-lot variability, and (6) may cause unwanted color variation in joint compound. Notwithstanding, attapulgite clay is typically used in the 1.5–3.5% by weight range.
There are benefits to the use of CMC in joint compounds. It does not entrap/entrain air, therefore pinholing post-sanding will not be a problem. Normally, CMC does not give sufficient slickness to a joint compound. However, when the attapulgite clay level is reduced, it does.
Although CMC has been used in tape joint compositions as a clay and/or mica replacement, it is difficult to accommodate in joint compound primarily because of the presence of low molecular weight cations, such as Mg++, Al+++ or Ca++ that are present. Even at relatively low concentrations, these cationic species can complex the CMC, resulting in gelation with undesirable joint compound rheology. Investigators have found ways to make specific CMC products perform acceptably under certain conditions.
U.S. Pat. No. 3,891,582 discloses a joint composition for wallboard sealing containing a resinous binder, limestone, clay, mica, lubricant, stabilizer and a thickener which are mixed with water to form a dope which is normally applied by troweling. A water-insoluble, fibrous CMC is used as an asbestos substitute.
U.S. Pat. No. 5,336,318 discloses clay-free joint compounds made with typical, industry-accepted thickeners in combination with an associative thickener. This composition cannot be used when dolomite limestone is used.
U.S. Pat. No. 5,382,287 discloses the use of superabsorbent CMC with a carboxymethyl degree of substitution (CMDS) in the 0.35–0.75 range and a degree of polymerization (DP) between 200 and 4000 to serve as a swelling agent and can replace clay and/or mica in a joint compound formulation when used in an amount of 0.01 to 0.5% by weight.
U.S. Pat. No. 5,512,616 discloses a joint compound composition using unborated HPGuar. It is specifically stated that underivitized guar cannot be used. No mention is made of reduced clay levels. The present invention makes use of underivitized guar when the attapulgite clay is reduced to levels well below standard practice, e.g., to 0.25–0.75% (1.5–2.5% is “typical”).
None of the prior art discloses the instant invention. Yet in spite of what was known in the prior art, a need still existed for simpler means for producing tape joint compounds.